Stage 2 Heads

Milzy's website says milling up to .020 free of charge for the stage 2 heads (they are originally .015). Would this really change anything (meaning would I need to buy anything else outside of the stage 2 package to fit)? I know it would increase airflow, but I didn't know if you could just have the heads milled to .020 and be alright or whatever... I figure there has to be a reason why they don't just make it .020 stock!